The musculoskeletal system of vertebrate organisms, including ourselves, is comprised of two basic systems, the axial and the appendicle. The postcranial axial system consists of the vertebra, ribs and associated muscles, and the appendicular system includes muscles and skeleton of the limbs and their respective girdles. In order for the organism to function properly in terms of locomotion, feeding and respiration, there must be proper integration of these two systems during embryonic development and growth. The cells that form all of the key parts of the musculoskeletal system arise from two populations of embryonic mesoderm, the somites and somatic lateral plate (abbreviated here as LP). The long term goal of this project is to determine how patterning genes influence cell behavior and morphological outcome as these two mesodermal cell populations mix to produce coordinated, functional anatomy. Our work to date has identified two dynamic domains in the developing chick embryo (Figure 3). The primaxial domain is populated only by cells from the somites. The abaxial domain is made up of myoblasts from the somites and connective tissue of the LP in which the muscles differentiate (Figures 1 & 3). We have termed the boundary between the two domains the Lateral Semitic Frontier (LSF), and postulated that cells on either side of the LSF are patterned by independent Hox codes resident in the connective tissue of the primaxial and abaxial domains (Burke and Nowicki, 2001, 2003). We suggest that the LSF is the site of early information exchange that is crucial for normal patterning of the vertebrate body. This proposal seeks to complete the mapping of the LSF in the chick embryo, extend mapping of the LSF into a mammalian model system (Aim 1), and explore functional implications of the LSF for body wall formation in both chick and mouse (Aim 2).